This invention relates to a method for preparing lamellar pigment materials by vacuum deposition and to lamellar pigment materials prepared by such methods. More particularly, it relates to an improved method whereby processing efficiencies and lamellar pigment materials of improved specular reflectivity are obtained.
The production of lamellar pigment materials by vacuum deposition techniques has been known and is described, for example, in U.S. Pat. Nos. 2,713,004 (issued July 12, 1955 to L. M. Greenstein); 3,123,489 (issued Mar. 3, 1964 to R. A. Bolomey et al); and in British Patent Specification No. 992,805 (published May 19, 1965). In general, such methods have been utilized in the production of nacreous pigments of lamellar or platelet shape exhibiting characteristic pearlescent or interference effects. Such pigment materials, because of their lamellar particle shape and high index of refraction, have found application in transparent substances of different index of refraction, such as plastics, as efficient reflectors of light from their platelet surfaces.
The production of lamellar pigment materials from high index of refraction substances such as zinc oxide, zinc sulfide, titanium dioxide, zirconium oxide and lead chloride by vacuum deposition has been described, for example, in the aforesaid U.S. Pat. No. 3,123,489. In general, the production of such lamellar pigment materials has involved the deposition of a suitable release-substrate material onto a surface, such as a rotating disc or endless belt, and the deposition thereon of the lamellar pigment-producing material. Typically, the lamellar pigment material has been removed by dissolving the release layer with a material which is a solvent for the release layer but a non-solvent for the lamellar pigment material.
Among the substances which have been reported as effective release-layer materials is sodium chloride. Sodium chloride, in part because of its ready availability, solubility in water and relative inertness to a variety of useful pigment-producing materials, has found special application as a release layer in the formation of lamellar pigment materials. The utilization of sodium chloride for such purposes is, however, attended by certain disadvantages. For example, sodium chloride, when evaporated with the aid of an electron beam, exhibits undesirable heat sensitivity which results in poor rate control and, therefore, deposition of a non-uniform coating of sodium chloride on the deposition surface. The deposition of a non-uniform layer of release coating leads to difficulty in the removal of desired pigment material by washing as the result of the deposition of pigment over areas where the sodium chloride layer is thin. The non-uniform deposition of the sodium chloride layer and consequent difficulties in removal of desired pigment material result in reduced yield of pigment material and re-use of a depositing surface, such as a Mylar web, is effectively negated.
The utilization of a source or charge of sodium chloride in the hearth of an electron beam gun in a vacuum deposition chamber is observed, in addition, to present a problem of cavitation or "tunneling". This is largely the result of the impinging of electrons of highest energy and in greatest number upon the center of the hearth containing the sodium chloride charge and this detracts from the uniform coating of the sodium chloride upon the deposition surface. The density of sodium chloride also imposes a limitation upon the conduct of extended deposition runs. A hearth utilized in vacuum deposition methods will generally be of fixed volume or capacity and density of the charge material will determine the weight of material which may be accomodated by the hearth. During extended runs, the sodium chloride charge tends to be rapidly exhausted.
Depending upon the nature of the substance utilized for the formation of a release layer, the physical properties of the resulting lamellar pigment material may also be effected. High specular reflectivity, for example, may be especially desired in some applications and the utilization of a release-layer material effective to provide such reflectivity in lamellar pigment materials will be especially advantageous.
Accordingly, it is an object of the invention to provide a process for preparing lamellar light-reflective pigment materials.
Another object of the invention is to provide a vacuum deposition process for the production of such pigment materials utilizing a substance which can be efficiently and effectively deposited as a release layer for the formation of such pigment material.
Still another object of the present invention is to provide a vacuum deposition process utilizing a substance which can be uniformly coated upon a deposition surface as a release layer for the formation of lamellar pigment material and which can be readily removed from the deposition surface by dissolution in a liquid which is a non-solvent for the lamellar pigment material.
Another object of the invention is to provide such a process effective to provide lamellar pigment material exhibiting improved specular reflectivity.
Other objects will become apparent from the description appearing hereinafter.